Six cotton cultivars were field tested for -years to determine dry matter accumulation and assimilate partitioning into vegetative and fruit organs under irrigated and water stressed environments in College Station, TX. The conversion efficiency of photosynthate into fruit and lint yield under the two water regimes was examined. The treatments consisted of two irrigation regimes: optimum water availability provided by irrigation, and no irrigation (dryland or rain-fed). The six cultivars were Tamcot CAMD-E (CAMD-E), Tamcot CD3H (CD3H), Tamcot HQ95 (HQ95), Lankart LX571 (LX571), Paymaster 404 (PAY404), and Deltapine 90 (DPL90).